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Spam is more than a nuisance - it's damaging to the environment, according to net security firm McAfee.

McAfee reasons junk email contributes to green house gas emissions because of the computer resources allocated to processing unwanted messages. Annual spam uses the equivalent electricity of 2.4m US homes or creates the carbon equivalent of 3.1m passenger cars using 2bn US gallons of gasoline, a McAfee-funded study calculates.

The security firm's Carbon Footprint of Spam report, released on Wednesday, is based on a study by climate-change consultant ICF International. Its figures are based on an estimate that 62tn spam emails were sent in 2008.

The international survey suggests the carbon footprint of a single spam message is 0.3 grams of CO2. More than half (52 per cent) of the energy consumption associated with junk mail comes from end-users deleting spam and searching for legitimate email, with spam filtering accounting for just 16 per cent of spam-related energy use. So, the argument goes, if more people used efficient inbox spam filters then even more energy might be saved.

Transmiting, processing and filtering these messages tied up computing resources that used an estimated 33bn kilo watt-hours (KWh) or 33 terawatt hours (TWh) of energy, equivalent to the electricity used in 2.4m US homes in the US. If every inbox was protected by a state-of-the-art spam filter, spam processing energy expenditure could be reduced by 75 percent or 25 TWh per year, the equivalent of taking 2.3m cars off the road, the survey suggests.

ICF reckons more than one-fifth of the annual energy used at a medium-size business on email is associated with spam. It estimates an average business email user is responsible for 131kg of CO2 per year in email-related emissions, one-fifth (or 22 per) is related to spam.

There are more leaps of logic in that reckoning than it's possible to adequately summarise. For starters, there's the idea that if they weren't processing spam messages then computer and routers would be idle. Gauging the financial impact of computer viruses is a notoriously difficult business, and we'd suggest the same applies to the cost - much less the impact on the environment - of spam.

McAfee's study recalls a January survey that suggests two Google searches take up a similar energy consumption as does boiling a kettle. A spokesman for the firm said that the earlier research did not inspire McAfee's probe, which came as a result of its wider research into cybercrime.

"As the world faces the growing problem of climate change, this study highlights that spam has an immense financial, personal and environmental impact on businesses and individuals," said Jeff Green, senior vice president of product development at McAfee Avert Labs.

"Stopping spam at its source, as well investing in state-of-the-art spam filtering technology, will save time and money, and will pay dividends to the planet by reducing carbon emissions as well." ®